Direct positive processes utilizing silver halide internal latent image emulsions containing high concentration of heterocyclic thione antifoggants

ABSTRACT

IN A KNOWN METHOD OFF SUCH WAVELENGTH TO PROVIDE PERMANENT RECT POSITVE IMAGES BY EXPOSING A LIGHT-SENSITIVE MATERIAL CARRYING A SILVER HALIDE EMULSION LAYER OF THE TYPE FORMINGA LATENT IMAGE PREDOMINANTLY IN THE INNER PART OF THE SILVER HALIDE GRAINS, I.E. A SO-CALLED INTERNAL IMAGE EMULSION, TREATING THE EXPOSED MATERIAL WITH A SURFACE DEVELOPER AND OVERALL EXPOSING THE MATERIAL DURING THIS TREATMENT TO ACTINIC LIGHT OF LOW INTENSITY, AN IMAGE HAVING MINIMUM DENSITY AND CONTRAST IS OBTAINED BY INCORPORATING WITHIN THE EMULSION LAYER AFOG-INHIBITING COMPOUND OF THE CLASS OF THE HETEROCYCLIC THIONE COMPOUNDS IN AN AMOUNT OF AT LEAST AOUT 0.1 MOLE PER 100 MOLE OF SILVER HALIDE AND WHICH IS SUBSTANTIALLY LARGER THAN NORMAL FOG-INHIBITING AMOUNTS. THE CONCEPT CAN BE APPLIED TO THE FORMATION OF BLACK AND WHITE IMAGES AS WELL AS TO COLORED IMAGES.

United States Patent 3,733,198 DRECT POSITIVE PROCESSES UTILIZING SILVER HALIDE INTERNAL LATENT IMAGE EMUL- SIONS CONTAINING HIGH CONCENTRATION OF HETEROCYCLIC THIONE ANTIFOGGANTS Gerard Laurens Vanreusel, Hove, Paul Desire Van Pee, Edegem, and Jules Maria De Laet, Mortsel, Belgium, assignors to Gevaert-Agfa N.V., Mortsel, Belgium No Drawing. Continuation-impart of abandoned application Ser. No. 546,528, May 22, 1966. This application Feb. 1, 1971, Ser. No. 111,706 Claims priority, application Great Britain, Apr. 30, 1965, 18,335/65 Int. Cl. G03c 5/24 U.S. Cl. 96-64 21 Claims ABSTRACT OF THE DISCLOSURE In a known method of photographically producing direct positive images by exposing a light-sensitive material carrying a silver halide emulsion layer of the type forming a latent image predominantly in the inner part of the silver halide grains, i.e. a so-called internal image emulsion, treating the exposed material with a surface developer and overall exposing the material during this treatment to actinic light of low intensity, an image having minimum density and contrast is obtained by incorporating within the emulsion layer a fog-inhibiting compound of the class of the heterocyclic thione compounds in an amount of at least about 0.1 mole per 100 mole of silver halide and which is substantially larger than normal fog-inhibiting amounts. The concept can be applied to the formation of black and white images as well as to colored images.

The present application is a continuation-in-part application of United States patent application Ser. No. 546,528, now abandoned.

The present invention relates to a novel method of producing photographic direct positive images and more particularly to a simple method according to which direct positive images of good quality are produced rapidly.

In German patent specification 749,864 a method is described of producing a photographic direct positive image that comprises the steps of exposing a silver halide emulsion layer to an object or image, predeveloping the thus exposed layer for a considerably long time, subjecting it to an overall exposure with actinic light of low intensity and finally developing the said emulsion layer to cause a direct positive image to be formed. The said predeveloping occurs preferably in a fine grain developer or flat-working developer and the said developing in an energetic developer. The silver halide emulsion layer for use in the process of said German patent specification must be such that its negative image sensitometric curve shows a maximum density that does not exceed the value 0.5.

In British patent specification 581,773 a method is described of producing a photographic direct positive image that comprises the steps of exposing to an object or image a light-sensitive silver halide emulsion layer, treating said emulsion layer to render it capable of forming a surface latent image and then subjecting it to a uniform exposure of sutficient intensity and time to cause a direct positive image to develop by treating it in a developer that will develop the surface latent image but will not or only slightly develop the first formed internal latent image. The silver halide emulsion layer used must be of the type in which the latent image isform'ed mostly or entirely inside the grains. According to the teachings of said British patent specification the light-sensitive silver halide emulsion layer that hasbeen exposed to an object 3,733,198 Patented May 15., 1973 uniform exposure of sufficient intensity and time to cause a direct positive image to be developed when the treatment is continued in the same or another surface developer.

The above methods of producing photographic direct positive images do not lead to the production of good quality images, particularly because the minimum density of direct positive image is not low enough. The fact that the minimum density is not low enough is due to an interference of the negative image, which forms in the silver halide emulsion layer in addition to the direct positive image, with the latter image.

It has now been found to produce a photographic direct positive image with a good maximum density and a clear minimum density by a method comprising the steps of:

Image-Wise exposing a light-sensitive material, which comprises a layer of a silver halide emulsion of the type that forms a latent image predominantly in the inner part of the silver halide grains,

Treating the light-sensitive material thus exposed with an aqueous energetic surface developing liquid, and

Overall exposing the light-sensitive material during the treatment stage to actinic light having an intensity, which is lower than a normal daylight intensity,

wherein a fog-inhibiting compound that inhibits fogging during the development stage and that belongs to the class of the heterocyclic thione compounds has been incorporated within said emulsion layer in an amount of at least about 0.1 mole per moles of said silver halide and which is substantially larger than normal fog-inhibiting amounts. In order to stabilize the photographic direct positive image obtained after said treating and overall exposure steps the material is generally fixed, e.g. in a usual fixing solution such as a thiosulphate solution, and washed, whereupon the material may be dried or glazed. Thus, the present invention provides a method of producing direct positive images of good quality in a rapid and simple way. This method is particularly suitable for producing continuous tone direct positive images but it is also appropriate for reproducing line originals.

The light-sensitive material of use according to the present invention generally comprises a support, for in stance a paper support or a hydrophobic film support that is either transparent or not, to which the light-sensitive silver halide emulsion layer is applied, if need be by means of an appropriate subbing layer.

The silver halide emulsion used in carrying out the method of this invention is of the type in which the latent image is formed predominantly in the inner part of the silver halide grains. This means an emulsion only few or none of which exposed grains are developable to silver by a developing solution which cannot act as a developer for latent image inside the grains, i.e. a so called surface developer, such as the following developing solution:

G p-Hydroxyphenyl glycine 10 Sodium carbonate (cryst) 100 Water to 1000 ccs.

and the exposed grains of which are well developable to silver by a developing solution which acts as a developer for latent image inside the grains, i.e. a so-called internal developer such as the following developing solution:

Hydroquinone 15 Monomethyl-p-aminophenol sulphate 15 Sodium sulphite (anhydrous) 50 Potassium bromide 10 Sodium hydroxide 25 Sodium thiosulphate (cryst.) 20

Water to 1000 ccs.

Any reference in the description and in the claims to a silver halide emulsion of the type that forms a latent image predominantly in the inner part of the silver halide grains is limited to a silver halide emulsion, a test layer of which, upon exposure to a light intensity scale for a fixed time between A and 1 second and development for 3 minutes at 20 C. in the above internal developer, exhibits a maximum density at least 3 times and preferably at least times, the maximum density obtained when an identical test layer of the said silver halide emulsion is equally ex posed and then developed for 4 minutes at 20 C. in the above surface developer. Said preference for a maximum density of at least 5 times as large particularly applies to the case the method of the invention is applied to the production of black and white direct positive images. When the method of the invention is applied to the production of color images, wherein the formed silver image is eliminated as will be explained detailedly further in the description, 3 times said maximum density will suffice in order to obtain good results.

The silver halide emulsions used according to the invention are generally not or only slightly chemically ripened silver halide emulsions, since the ability of forming the surface latent image increases with the degree of chemical ripening.

Silver chlorobromide emulsions comprising at least 20 mole percent of silver bromide and pure silver bromide emulsions have proved to be specially suitable for the purpose of the invention. These emulsions may contain up to 5 mole percent of silver iodide relative to the total amount of silver halide. Washed as well as unwashed silver halide emulsions may be used.

The silver halide emulsions for use in the present process are generally gelatino silver halide emulsions. However, the gelatin may at least partly be replaced, e.g., by another protein, a hydrophilic not proteinaceous colloid, e.g. a polyvinylpyrrolidone or a synthetic polymer applied from an aqueous dispersion, i.e., a latex, e.g. a polyethyl acrylate latex. The presence of such other binders often has also a favourable photographic effect on the formation of the direct positive image. For instance, the addition of polyvinylpyrrolidone and of said polyethyl acrylate latex in most cases increases the maximum density of the direct positive image.

The light-sensitive silver halide emulsions used according to the invention may be prepared according to any technique known in the art. A method according to which emulsions are prepared, which have proved to be particularly suitable for the purpose of the invention is the so-called conversion method. According to this method a more soluble silver halide is converted into a less soluble silver halide e.g. a silver chloride emulsion is converted in the presence of water-soluble bromide and occasionally iodide, the amounts of which are adapted in view of the final composition aimed at, into a silver chlorobromide or a silver bromide emulsion which may occasionally comprise small amounts of silver iodide. This conversion is carried out preferably very slowly by several consecutive steps. Another technique according to which suitable emulsions for use according to the invention may be obtained has been described in the British patent specification 1,011,062. For further specifications of the nature and preparation of light-sensitive silver halide emulsion layers suitable for use according to the present invention we refer to the specific examples given below.

4 Into the silver halide emulsion layer a fog-inhibiting compound is incorporated that inhibits fogging during the development stage and that belongs to the class of the heterocyclic thione compounds. This compound is incorporated within said emulsion layer in an amount of at least about 0.1 mole per moles of silver halide and which is substantially larger than normal fog-inhibiting amounts. The presence of this large amount of fog-inhibiting compound causes the favorable elfect of the invention, 'viz. the production of a direct positive image showing a low minimum density and an improved contrast. According to a preferred embodiment the fog-inhibiting compound is incorporated Within the silver halide emulsion layer by adding it to the liquid silver halide emulsion, generally just before coating. However, the fog-inhibiting compound may also be incorporated within the emulsion layer by diffusion from or through another layer, which is in water-permeable relationship with the emulsion layer. This diffusion occurs automatically and immediately when the silver halide emulsion is applied to a water-permeable layer containing the fog-inhibiting compound or when an aqueous coating liquid containing said compound is applied to the silver halide emulsion layer or to a waterpermeable layer present on top of the emulsion layer. In any case the fog-inhibiting compound has to be present in the silver halide emulsion layer before the aqueous energetic surface developer is applied to the image-wise exposed light-sensitive material. Adding the fog-inhibiting compound to the developing liquid instead of incorporating it within the silver halide emulsion layer as described above results in a considerable decrease of the maximum density of the direct positive image as will be shown in Example 4 hereinafter. Proceeding in this way is outside the scope of this invention.

As there has already been mentioned above the foginhibiting compound is applied in an amount which is substantially larger than normal fog-inhibiting amounts, i.e. in an amount, which is normally not incorporated within a silver halide emulsion layer because it would too strongly desensitize the light-sensitive material. The surprising fact, however, in the process of the present invention is that these large amounts of fog-inhibiting compound do not lower the sensitivity of the direct positive image but in most cases even enhance it and also do not prevent the formation of a direct positive image with a good maximum density.

Specific examples of suitable fog-inhibiting compounds belonging to the class of the heterocyclic thione compounds are:

4-phenyl-A -thiazoline-Z-thione,

4,5 -diphenyl-A -thiazoline-Z-thione,

4,5-diphenyl-A -oxazoline-2-thione,

benzothiazoline-Z-thione,

4-phenylA -oxazoline-Z-thione,

4,5 -diphenyl-A -oxazoline-Z-thione,

4-phenyl-A -imidazoline-Z-thione,

4,5 -diphenyl-h -imidazoline-Z-thione,

l-me thyl-benzimidazoline-Z-thione,

1-phenyl-imidazolidine-2-thione,

3-phenyl-A l,2,4-thiadiazoline-S-thione,

4-phenyl-A -1,2,4-triazoline-5-thione,

3 -phenyl-A -1,2,4-triazoline-5-thione,

1-methyl-2-tetrazoline-5 -thione,

l-phenyl-Z-tetrazoline-S -thione,

1- l-naphthyl) -2-tetrazoline-5-thi0ne,

1- (Z-naphthyl) -2-tetrazoline-5-thione,

1- (9-anthryl) -2-tetrazoline-5-thione,

1- 3,4-dichlorophenyl) -2-tetrazoline-5-thione,

l- (o-methoxy-phenyl -2-tetrazoline-5-thione,

1- (o-biphenylyl) -2-tetrazoline-5-thione,

1- (p-biphenylyl -2-tetrazoline-5-thione, and

1- Z-naphthyl) -4,4,6-trimethyl-1,2,3,4-tetrahydropyrimidine-Z-thione.

The best results are obtained with rfive-membered heterocyclic thione compounds carrying an aryl substituent or a fused-on aromatic group. Most of the specific examples of suitable heterocyclic thione compounds listed hereinbefore belong to this subclass.

It will be understood that any reference in the description and in the claims to fog-inhibiting compounds of the class of the heterocyclic thione compounds is intended to refer also to the tautomeric structures of said com:- pounds and to salts of said-compounds or occasionally of said tautomeric structures. Mixtures of two or more foginhibiting compounds may also be employed.

In a first step for the preparation of the direct-positive image the light-sensitive material is image-wise exposed to the original to be reproduced. This exposure can be either a high-intensity exposure such as flash exposure or a normal-intensity exposure such as day-light exposure as well as low-intensity exposure such as an exposure by means of a printer or an exposure of still lower intensity, the exposure-time being dependent on the sensitivity of the light-sensitive material for the direct positive image formation which sensitivity according to the present invention may be a camera speed as well as a lower sensitivity.

In a further step of the direct positive image formation according to the present invention the image-wise exposed light-sensitive material is treated with an aqueous energetic surface developing liquid. This treatment may be carried out according to any technique known in the art, e.g. by soaking or by wetting only one side of the light-sensitive material, e.g. by means of a lick roller, by spreading a paste, e.g. contained in a pod or by spraymg.

The developer of use in the process of the invention must be a surface developer i.e. a developer that contains no or at least no effective amount of solvents for silver halide. In this case by solvents for silver halide there are meant particularly strong solvents for silver halide such as water-soluble thiocyanates, thiosulphates, ammonia, etc. Indeed, compounds that may be considered to be but very weak solvents for silver halide, such as sodium sulphite in case the silver halide is sliver bromide, silver chlorobromide, or silver bromoiodide, may be present in the developer.

Further the developer has to be an energetic developer. The high energy required can be realized by taking, e.g. one or more of the following measures:

using a developer with relatively high pH-value,

working at an elevated temperature,

using a developer wherein the concentration of the various compounds present is relatively high, and

incorporating developing activators, such as cyclohexylamine, polyethylene glycols and derivatives in the developer.

The choice of the developing substances too determines the energy of the developer. Examples of suitable developing substances are among others the combination of l-phenyl-S-pyrazolidinone with hydroquinone and the combination of monomethyl-paminophenol sulphate with hydroquinone. In general there may be said that the more energetic the developer is the better the result obtained will be.

For carrying out the method of the invention the developing agents may also be incorporated partially or even completely into the light-sensitive material. This incorporation may be accomplished during the preparation stage of the material or later on by means of a processing liquid, with which the light-sensitive material is wetted prior to the development of the direct positive image. In doing so the energetic surface developer can be reduced to a mere alkaline liquid substantially free from developing agents. Such an alkaline aqueous liquid, often called activator, offers the advantage of having a longer activity, i.e. of being less rapidly exhausted.

The developer (occasionally activator) may be sup plied in an amount which suflices for the treatment of exactly one foil of light-sensitive material. A bath of this type offers the advantage that ageing and contamination of the bath composition are avoided.

While the image-wise exposed light-sensitive layer is present in the developer or is moistened therewith in some other way the said light-sensitive material is overall and generally uniformly exposed to actinic light of low intensity, i.e. of an intensity, which is lower than a normal daylight intensity. This exposure may begin simultaneously with the treatment of the light-sensitive material with developer but occurs favourably a little later for instance from 5 to 30 seconds later. The duration of the secondary or overall exposure is not very critical and may amount from 10 seconds to many minutes according to the nature of the light-sensitive material, the composition and the temperature of the developer, the intensity of the light-source employed, etc. In many cases the overall exposure lasts till the direct positive image is completely developed. This offers the additional advantage of being able to follow the development and stop it when the direct positive image formation is thought to have reached its optimum value. The density of the direct positive image can be made more intense or reduced locally during the second or overall exposure by the well known technique of burning in and dodging. In general the total development of the direct positive image does not take much time and in most cases may be carried out in from 1 to 5 minutes. The uniformity of the said overall exposure to actinic light of low intensity is mostly achieved by interposing between the light-source and the light-sensitive material to be exposed a light-diffusing member that transmits at least part of the light rays of the spectral region for which the light-sensitive material is sensitive. The required low intensity of the light rays, which reach the light-sensitive material, can be obtained by adapting the light-source and/ or the light-diffusing member.

As already stated above, the direct positive images obtained according to the present invention are of good quality in the high-light areas as well as in the dark areas.

In the method according to the invention the maxi mum or minimum density of the direct positive image can still be improved and the characteristics of the lightsensitive material can be altered by using all kinds of ingredients which are geneally known in the art of emulsion preparation and some of which will be set out specifically hereinafter.

Most of these ingredients are preferably incorporated into the light-sensitive material itself, in effective contact with the silver halide emulsion layer and favourably in the latter layer itself. Many of these ingredients, however, may be incorporated into the energetic surface developer with the same favourable result. Combinations of two or more of said ingredients, of course, may also be used.

Among the said ingredients may be mentioned some binders for at least partially replacing gelatin as a binder for the silver halide grains and referred to already above in the description, futher optical sensitizers which increase the maximum density of the direct positive image, and moistening agents such as polyalkylene glycols, e.g. polyethylene glycols, sulphonated fatty acids, saponine and the like. Still further ingredients are mentioned later on in the description.

The process according to the present invention is not restricted to black and white photography; it may also be applied for the preparation of direct positive colour images by carrying out the development in an energic colour developer in the presence of a colour coupler and by bleaching away the silver developed. The light-sensitive material, of course, may be a multilayer colour material each layer of which absorbs a particular part of the spectrum and for which layers appropriate colour couplers are provided. Besides their developing or coupling action may colour developing compounds and colour couplers exert an advantageous effect on the mechanism of the production itself of the direct positive image in that they contribute to a higher maximum and/or a lower minimum density.

According to the method of the invention a negative image is formed in the silver halide emulsion layer in addition to the direct positive image, just as is the case with the Sabattier effect. In the latter case, however, the positive and negative image overlap with each other, whereas according to our invention there is a clear separation between the direct positive image and the negative image that forms in a lower sensitivity range. This separation is clearest when the image-wise exposure is a high-intensity exposure, e.g. a flash exposure, and is less clear according as the intensity of the image-wise exposure is lower. Nevertheless, excellent results still can be obtained with a normal day-light exposure and even with an exposure of still lower intensity, e.g. an exposure by means of a printer.

In the light-sensitive material for use in the method of the invention a negative image in the low sensitivity range can also be obtained by developing this material after image-wise exposure in a surface developer without subjecting it to a second or overall exposure.

By developing the light-sensitive material for use according to the present invention after image-wise exposure in an internal developer a negative image is obtained that lies in a sensitivity range only slightly lower than that of the direct positive image obtained with a same material according to the process of the present invention.

Thus the direct positive image obtained by proceeding according to the method of the invention lies in the highest sensitivity range.

The following are examples illustrating the process of the present invention.

EXAMPLE 1 A gelatino silver bromide emulsion that mainly forms internal latent image and little external latent image is prepared through conversion of a silver chloride emulsion by very slowly adding to said emulsion whilst stirring a 35% aqueous potassium bromide solution in an amount that is 70% higher than the amount, which is theoretically needed for converting all the silver chloride. Then the emulsion is kept for 1 hour at 60 C. After the emulsion has been cooled and allowed to gel for 6 hours the emulsion is noodled. The noodles are washed for 1 hour with water (10 litres of water per minute). By heating the washed noodles a liquid silver bromide emulsion is obtained containing 50 g. of gelatin and 0.4 mole of silver bromide per kg.

Per kg. of this liquid emulsion are added 10 ccs. of a solution in ethanol of 1-(o-methoxyphenyl)-2-tetrazoline-S-thione.

The light-sensitive emulsion is coated practically immediately after the addition of the said solution in ethanol on a baryta coated paper support of 130 g./sq. m in such a way that an amount of silver halide equivalent to 4 g. of silver nitrate is present per sq. m of light-sensitive material.

The emulsion layer is overcoated with a conventional hardened gelatin antistress layer.

The light-sensitive silver halide material obtained is image-wise exposed for sec. through a step-wedge by means of a flash exposure of 130 lux sec.

A first strip of the exposed light-sensitive material is developed for 4 min. at 20 C. in a surface developer of the following composition:

G. Hydroquinone 15 l-phenyl-3-pyrazolidinone 1 Anhydrous sodium carbonate 30 Anhydrous sodium sulphite 40 Water up to 1000 ccs. Sodium hydroxide till pH 11.

The strip is then fixed, rinsed and dried in the conventional way. A very faint negative image of the original to be reproduced is obtained.

A second strip of the above exposed light-sensitive material is treated in the same way as the first strip with the difference, however, that 30 sec. after the development has started the light-sensitive material is overall exposed for the remaining time of the development through a gray filter having a density of 1.5 by means of a 15 watt lamp placed at a distance of 75 cm. from the light-sensitive material. A direct positive image of the original is obtained having a good maximum density and a very low minimum density and forming in a higher sensitivity range than that wherein the negative image of the first strip is formed. As compared with a direct positive image obtained in the same manner but with a light-sensitive material containing no or only a minor amount of fog-im hibiting compound the direct positive image obtained according to the method of the invention as described in this example shows a much lower minimum density and a maximum density that has not lowered to the same extent as the said minimum density but to a much smaller degree.

When a third strip of the above image-wise exposed light-sensitive material is treated in the same way as the first strip with the difference however, that the developer comprises in addition to the above ingredients 5 g. of anhydrous sodium thiosulphate per litre so that it is an internal developer, 2. negative image of the original is obtained having a higher maximum density than that obtained on the first strip and forming in a higher sensitivity range.

EXAMPLE 2 Example 1 is repeated with the difference, however, that in the preparation of the silver halide emulsion the amount of 1-(o-methoxyphenyl)-2-tetrazoline-S-thione is replaced by 20 ccs of a 2.5% solution in methyl glycol of 5- 3-phenylureido 1 H-benzotriazole.

Similar results as described in Example 1 are obtained.

EXAMPLE 3 A gelatino silver bromide emulsion that mainly forms internal latent image and little external latent image is prepared through conversion of a silver chloro-bromide emulsion (10 mole percent of bromide) by very slowly adding to said emulsion whilst stirring a 35% aqueous potassium bromide solution in an amount that is higher than the amount which is theoretically needed for converting all the silver chloride. The emulsion is then kept for 1 h. at 60 C.

After the emulsion has been chilled and allowed to gel for 6 hours the emulsion is noodled. The noodles are washed for 1 hour with water (10 litres of water per minute). By heating the washed noodles a liquid silver bromide emulsion is obtained containing 50 g. of gelatin and 0.4 mole of silver bromide per kg.

To this liquid emulsion an appropriate colour coupler for yellow is added in the common concentration whereupon l-(o-methoxyphenyl)-2-tetrazoline-5-thione is added from a 5% solution in ethanolzto a first part of the emulsion 5 ccs./kg, to a second part 10 ccs./kg. and to a third part 20 ccs./kg.

Practically immediately thereafter each of these lightsensitive emulsions is coated on a baryta coated paper support of g./sq. m in such a way that an amount of silver halide equivalent to 4 g. of silver nitrate is present per sq. m of light-sensitive material.

The emulsion layer of each of the materials is overcoated with a conventional hardened gelatin antistress layer.

The three silver halide materials obtained are further treated in a completely identical way. Each material is image-wise exposed for sec. through a step-wedge by means of a flash exposure of 130 lux sec. Then, each material is developed for 3 min. at 26 C. in a developer of the following composition:

Water up to 1000 ccs.

Each light-sensitive material is first developed for 30 sec. in the dark and then for the remaining time while being overall exposed through a grey filter with a density of 2.1 by means of a 15 watt lamp placed at a distance of 70 cm. from the light-sensitive material.

After the development each material is treated in a stop bath, bleached, fixed and rinsed With water as is the case with conventional photographic colour material.

A yellow positive image of the step-wedge is obtained in each of the three materials. The three images all have a high maximum density and a very low minimum density, which is the lower the more fog-inhibiting compound is present. As compared with a direct positive image obtained in the same manner but with a light-sensitive material containing no or only a minor amount of foginhibiting compound the direct positive images obtained according to the method of the invention as described in this example show a much lower minimum density and a maximum density that has not lowered to the same extent as the said minimum density but to a much smaller degree.

EXAMPLE 4 A gelatino silver bromide emulsion that forms a latent image predominantly in the inner part of the silver halide grains was prepared through conversion of a silver chloride emulsion by very slowly adding to said emulsion Whilst stirring a 35% aqueous potassium bromide solution in an amount that was 70% higher than the amount theoretically necessary for converting all the silver chloride. The emulsion was then kept for 1 hour at 60 C. .After the emulsion had been chilled and allowed to gel for 6 hours it was noodled. The noodles were washed for 1 hour with Water (10 litres of water per minute). By heating the Washed noodles a liquid silver bromide emulsion was obtained containing 50 g. of gelatin and 0.4 mole of silver bromide per kg.

Per kg. of the liquid emulsion thus obtained the following solutions were added:

Ccs.

35% aqueous solution of potassium bromide 25 aqueous solution of potassium iodide 12 5% solution of l-(o-methoxyphenyl)-2-tetrazoline S-thione in ethanol 20 After the addition of an optical sensitizer and some other usual coating aids, the silver halide emulsion was applied to a baryta-coated paper support provided with a thin layer of cellulose nitrate. It was applied in such a way that an amount of silver halide equivalent to 5.5 g. of silver nitrate was present per sq. m. of the lightsensitive material.

Finally, the emulsion layer was provided with a conventional hardened gelatin antistress layer.

The light-sensitive material was image-wise exposed for 1 second through a step-wedge having a wedge constant of 0.1 by means of a low intensity lamp. It was then 10 developed for 65 seconds at 30 C. in a developer having the following composition:

Hydroquinone g 15 1-phenyl-3-pyrazolidinone g 1 Sodium sulphite (anh.) g 70 Sodium carbonate (anh.) g 30 Trisodium salt of ethylenediamine tetraacetic acid L g 1 40% aqueous sodium hydroxide ccs 16 Water up to ccs 1000 25 seconds after the development had been started the light-sensitive material was overall exposed for the remaining time of the development through a grey filter having a density of 2.7 by means of a 15 watt lamp placed at a distance of 70 cm. from the light-sensitive material. The light-sensitive material was then fixed, rinsed and dried in the conventional way. The direct positive image obtained shows a good gradation, a low minimum density (0.32) and a high maximum density (1.66).

A second direct positive image was prepared in an analogous manner with the difference, however, that 1- (o-methoxyphenyl)-2-tetrazoline-5-thione was omitted from the light-sensitive material and instead thereof Was incorporated into the developer by adding 25 cos. of a 5% solution in ethanol per liter of developer. The amount of fog-inhibiting compound in the developer had been chosen in such a Way that the most favorable result was obtained. The direct positive image thus obtained shows a somewhat lower minimum density (0.24) indeed, but also shows a considerably lower maximum density (1.25) and a lower gradation.

EXAMPLE 5 A series of direct positive images Was prepared in an analogous manner as the first direct positive image described in Example 4. However, instead of adding 20 cos. of a 5% solution of 1-(o-methoxyphenyl)-2-tetrazoline-5-thione in ethanol per kg. of the liquid silver bromide emulsion, the fog-inhibiting compounds listed in the table hereinafter were added to the liquid emulsion in the given amounts. The following results were obtained:

TAB LE Amount of fog-inhibiting compound in mg. per kg. Maxi- Miniliquid silver mum mum bromide denden- A Fog-inhibiting compound emulsion sity sity density l-(o-methoxy-phenyl)-2-tetrazoline-S-thione 750 1. 75 0. 31 1.44 4-phenyl-A thiaZ0line-2-thione. 693 l. 55 0. 26 1. 29 4,5-diphenyl-A -thiazoli 2- thioue 968 1. 00 0. 22 0. 78 B enZothiaZoline-2-th 600 1. 44 0. 23 1. 21 4-phenyl-N-oxaZoline-2-thione 636 1. 22 0. 22 1. 00 4,5-diphenyl-A -oxaZoline-2- tione 910 1. 50 0.23 1. 27 -phenyl-A -imidaZoline-2-thione 634 l. 05 0. 23 0. 82 4,5-diphenyl-A imidaZoline-2- thione 908 0. 94 0. 21 0. 73 l-methyl-benzimidazoli thione 509 1. 48 0. 25 1. 23 1-phenyl-imidaZolidine-Z-thioue- 640 1. 09 0. 29 0. 3-phenyl-A -1,2,4-thiadiazoline- 5-thione 698 l. 11 0.33 0. 78

We claim:

1. In a method of producing a photographic direct positive image that comprises the steps of image-wise exposing a light-sensitive material, which comprises a layer of a silver halide emulsion of the type that forms a latent image predominantly in the inner part of the silver halide grains, treating the light-sensitive material thus exposed with an aqueous energetic surface developing liquid, and overall exposing the light-sensitive material during the treatment stage to actinic light having an intensity, which is lower than a normal daylight intensity, whereby a direct positive image is developed in said emulsion layer,

the improvement which comprises incorporating within said emulsion layer a fog-inhibiting compound inhibiting fogging during the development of said direct positive image, wherein the fog-inhibiting compound belongs to the class of the heterocyclic thione compounds and is present in said emulsion layer in an amount of at least about 0.1 mole per 100 moles of said silver halide and which is substantially larger than normal fog-inhibiting amounts.

2. A method according to claim 1, wherein the foginhibiting compound is a five-membered heterocyclic thione compound carrying an aryl substituent or a fusedon aromatic group.

3. A method according to claim 1, wherein the foginhibiting compound is l-(o-methoxy-phenyl)-2-tetrazoline-S-thione.

4. A method according to claim 1, wherein the silver halide emulsion is a silver bromide emulsion or a silver chlorobromide emulsion comprising at least 20 mole percent of silver bromide and containing at most 5 mole percent of silver iodide relative to the total amount of silver halide.

5. A method according to claim 4, wherein the silver halide emulsion is prepared by conversion from a more soluble silver halide emulsion.

6. A method according to claim 1, wherein the silver halide emulsion layer includes a moistening agent.

7. A method according to claim 6, wherein said moistening agent is a polyalkylene glycol.

8. A method according to claim 1, wherein the silver halide emulsion layer includes polyvinylpyrrolidone.

9. A method according to claim 1, wherein the silver halide emulsion layer includes an aqueous dispersion of a synthetic polymer.

10. A method according to claim 9, wherein said synthetic polymer is polyethyl acrylate.

11. In a method of producing a colored photographic direct positive image that comprises the steps of:

image-wise exposing a light-sensitive material, which comprises a layer of a silver halide emulsion of the type that forms a latent image predominantly in the inner part of the silver halide grains, treating the light-sensitive material thus exposed with an aqueous energetic color surface developing liquid in the presence of a color coupler,

overall exposing the light-sensitive material during the treatment stage to actinic light having an intensity, which is lower than a normal daylight intensity, whereby a direct positive image is developed in said emulsion layer, and

bleaching away the silver developed, the improvement which comprises incorporating within said emulsion layer a fog-inhibiting compound inhibiting fogging during the development of said direct positive image, wherein the fog-inhibiting compound belongs to the class of the heterocyclic thione compounds and is present in said emulsion layer in an amount of at least about 0.1 mole per moles of said silver halide and which is substantially larger than normal fog-inhibiting amounts.

12. A method according to claim 11, wherein the foginhibiting compound is a five-membered heterocyclic thione compound carrying an aryl substituent or a fusedon aromatic group.

13. A method according to claim 11, wherein the foginhibiting compound is 1-(o-methoxy-phenyl)-2-tetrazoline-S-thione.

14. A method according to claim 11, wherein the silver halide emulsion is a silver bromide emulsion or a silver chlorobromide emulsion comprising at least 20 mole percent of silver bromide and containing at most 5 mole percent of silver iodide relative to the total amount of silver halide.

15. A method according to claim 14, wherein the silver halide emulsion is prepared by conversion from a more soluble silver halide emulsion.

16. A method according to claim 11, wherein the silver halide emulsion layer includes a moistening agent.

17. A method according to claim 16, wherein said moistening agent is a polyalkylene glycol.

18. A method according to claim 11, wherein the silver halide emulsion layer includes polyvinylpyrrolidone.

19. A method according to claim 11, wherein the silver halide emulsion layer includes an aqueous dispersion of a synthetic polymer.

20. A method according to claim 19, wherein said synthetic polymer is polyethyl acrylate.

21. A method according to claim 11, wherein several silver halide emulsion layers constitute a multilayer color material each layer of which absorbs a particular part of the spectrum and for which layers appropriate colour couplers are provided.

References Cited UNITED STATES PATENTS 2,497,876 2/1950 Fallesen et al 96-64 NORMAN G. TORCHIN, Primary Examiner W. H. LOUIE, 1a., Assistant Examiner US. Cl. X.R. 

